This invention relates to a lens system with plastic lenses and more particularly to a zoom lens which can optically correct shifting of an image forming plane caused by deformation and optical characteristic changes of plastic lenses due to temperature changes.
With a view of reducing weight and cost of optical lenses, plastic lenses have currently been put into practice. Mainly used as materials of the plastic lenses are polymers such as methylmetaacrylate or styrene (methylmetaacrylate polymer and styrene polymer are respectively abbreviated as acryl and styrene hereinafter) and copolymers. These materials, however, have optical constants having larger dependency upon temperature changes than those of glass materials. More particularly, in these plastic lens materials, the temperature dependent change rate of refractive index is about 10.sup.-4 /.degree.C. which is about 100 times as large as that of glass material and the temperature dependent change rate of coefficient of linear expansion is about 10.sup.-4 /.degree.C. which is about 10 times as large as that of glass material. Therefore, the focal length of each plastic lens constituting an optical lens system varies with changes in temperature more greatly than that of glass lenses does, with the result that the position of an image forming plane mainly shifts and an image is degraded. On the other hand, the allowable amount of shifting of the image forming plane due to temperature changes is determined depending on the depth of focus. For example, since a video camera with a 2/3" imaging device has an allowable lateral aberration of about 40 .mu.m, an F-number (hereinafter referred to as "F") 5 lens provides a depth of focus of about 200 .mu.m corresponding to the 40 .mu.m lateral aberration. An F=1.4 lens of larger aperture, however, provides only a depth of focus of about 50 .mu.m corresponding to the 40 .mu.m lateral aberration. Incidentally, a prior art lens system simply substituting plastic lenses for glass lenses suffers from shifting of an image forming plane which amounts to 300 to 500 .mu.m under the influence of a temperature change of 30.degree. C. in practical use. This means that only the depth of focus provided by a lens of the order of F=10 can fall within the allowable range of 40 .mu.m lateral aberration. Under the circumstances, design for large-aperture plastic lenses must take into consideration and solve the problem of shifting of the image forming plane due to temperature changes.
The temperature dependent shifting of the image forming plane in a plastic lens will now be described in greater detail. Where a plastic single lens has an amount of temperature dependent change of refractive index represented by .alpha., an amount of temperature dependent change of coefficient of linear expansion represented by .beta., a refractive index of N, a radius of curvature of R at front and back surfaces, a focal length of f, and an amount of temperature dependent change of focal length represented by .DELTA.f corresponding to a change of temperature .DELTA.T, the following relation stands: ##EQU1## where ##EQU2## Assuming that -.DELTA.f/f (temperature change dependent variance of focal length) is V.sub.T at .DELTA.T=30.degree. C., a lens system undergoes shifting of the image forming plane by Ath which is expressed as, EQU Ath=f.sup.2 .SIGMA.h.sub.i.sup.2 /(f.sub.i .multidot.V.sub.Ti) (1)
where h.sub.i, f.sub.i and V.sub.Ti respectively represent a height of a pheripheral light beam, a focal length and a value of V.sub.T related to an i-th single lens. To eliminate the temperature dependent shifting of the image forming plane, Ath=0 must be satisfied in connection with equation (1).
However, the focal length of radius of curvature of a single plastic lens or each of the plastic lens bonded together is constrained by optical design and for this reason, it was difficult to satisfy Ath=0 in equation (1) by adjusting the focal length or radius of curvature of each plastic lens for the sake of correcting shifting of the image forming plane caused by deformation and optical characteristic changes of plastic lenses due to temperature changes.